CRED: Use RCU to access another task's creds and to release a task's own creds

Use RCU to access another task's creds and to release a task's own creds.
This means that it will be possible for the credentials of a task to be
replaced without another task (a) requiring a full lock to read them, and (b)
seeing deallocated memory.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
index 10715d1330b9..c86303638235 100644
--- a/security/selinux/selinuxfs.c
+++ b/security/selinux/selinuxfs.c
@@ -95,13 +95,18 @@ extern void selnl_notify_setenforce(int val);
 static int task_has_security(struct task_struct *tsk,
                              u32 perms)
 {
-        struct task_security_struct *tsec;
-
-        tsec = tsk->cred->security;
+        const struct task_security_struct *tsec;
+        u32 sid = 0;
+
+        rcu_read_lock();
+        tsec = __task_cred(tsk)->security;
+        if (tsec)
+                sid = tsec->sid;
+        rcu_read_unlock();
         if (!tsec)
                 return -EACCES;
 
-        return avc_has_perm(tsec->sid, SECINITSID_SECURITY,
+        return avc_has_perm(sid, SECINITSID_SECURITY,
                             SECCLASS_SECURITY, perms, NULL);
 }